The present invention relates to voltage reference circuits for use in integrated circuits and more particularly to a true bandgap voltage reference useful in CMOS integrated circuits.
References known to the present applicants and believed to be relevant to the present invention include the following publications.
R. J. Widlar, "New Developments in IC Voltage Regulators", IEEE Journal of Solid State Circuits, Vol. SC-6 pp. 2-7, February 1971;
K. E. Kuijk, "A Precision Reference Voltage Source", IEEE Journal of Solid State Circuits, Vol. SC-8, pp. 222-226, June 1973;
A. P. Brokaw, "A Simple Three Terminal IC Bandgap Reference", IEEE Journal of Solid State Circuits, Vol. SC-9, pp. 388-393, December 1974;
E. A. Vittoz, et al, "A Low Voltage CMOS Bandgap Reference", IEEE Journal of Solid State Circuits, Vol. SC-14, pp. 573-577, June 1979; and
G. Tzanateas, et al, "A CMOS Bandgap Voltage Reference", IEEE Journal of Solid State Circuits, Vol. SC-14, pp. 655-657, June 1979.
The first three of the above publications teach the basic concept of bandgap voltage references and the need for such precise circuits. Each of the particular circuits provided in these publications is quite suitable for fabrication on typical bipolar integrated circuits where isolated transistors are available. Each of the circuits is based on the principle that by the proper combination of a base-emitter voltage, V.sub.BE, of one transistor with the difference in base-emitter voltages, .DELTA.V.sub.BE, of two transistors operating at different current densities a reference which is stable over a wide range of temperatures can be achieved. By proper scaling the positive temperature coefficient of the .DELTA.V.sub.BE term will balance the negative temperature coefficient of the V.sub.BE term itself. The circuit taught by Widlar provides a stable voltage reference based on this principle but is generally limited to an output voltage no greater than the bandgap voltage itself.
The circuits taught in the Brokaw publication provide a resistive divider in the feedback loop to the transistors establishing the reference voltage so that the actual output voltage may be essentially any scaler of the basic bandgap voltage.
The Kuijk publication teaches yet another bandgap voltage source similar in principle to the Brokaw and Widlar devices, but employing two diode connected transistors. The output voltage for this circuit is generally limited to the bandgap voltage or some integral multiple thereof which may be achieved by stacking multiple references.
The circuits taught in each of these three publications provide very good reference voltages that are generally suited for use only on standard bipolar integrated circuits. It is also desirable to provide accurate voltage references in MOS type integrated circuits. These first three types of circuits are however not suitable for conventional MOS processing since they all require the use of bipolar transistors having isolated collectors. Such transistors can be provided in MOS circuits but only at the expense of added processing steps so that the overall process could not be considered a conventional MOS or CMOS process.
The last two of the above publications address the problem of providing good voltage references in CMOS integrated circuits without requiring the use of additional processing steps.
The circuits of the last two references both rely on a circuit known as a proportional to absolute temperature circuit constructed entirely of MOS transistors which provides an output which is combined with the base-emitter drop of a single bipolar transistor formed on a CMOS integrated circuit. While such an arrangement is conceptually sound, it is susceptible to surface effects such as surface traps and contamination since the MOS devices are surface devices. In addition, the MOS proportional to absolute temperature circuitry relies on a weak inversion region of operation of the MOS devices over the temperature range of interest thereby requiring careful control of operating conditions. In the Tzanateas, et al configuration the output is generally limited to the bandgap voltage and stacking of the circuits to obtain other reference potentials would be difficult since the negative terminal of the output voltage is floating. The circuit provided by Vittoz, et al operates at low input voltages and with low current requirements but it is sensitive to resistor ratios, the ratios of MOS device dimensions are large, and the bias point is offset by leakage currents even at room temperature. In any case, as noted in the last sentence of the Vittoz publication, the voltage references thus far provided on CMOS chips are not nearly as accurate as true bipolar bandgap references and this reduction in quality has been accepted as a tradeoff to avoid additional processing steps in the CMOS circuits.